The production of fibers by meltspinning is widely practiced throughout industry. In general, molten polymer is extruded through a plurality of fine orifices to provide a plurality of fine polymer streams which are then quenched and attenuated. Attenuation or drawing can be accomplished in various ways including mechanically and pneumatically. Mechanical drawing involves the use of precisely controlled filament winding apparatus wherein the speed of the winding apparatus determines the drawing force applied to the quenched fibers. In the pneumatic process, the fibers are passed through a zone of rapidly moving gases, typically air, which apply attenuation force to the filaments.
Polyolefin polymers, particularly polypropylene (both isotactic and syndiotactic) and its copolymers and terpolymers, have been used extensively for meltspinning of fibers. Polyolefins are relatively inexpensive and can provide fibers in a wide range of deniers, strength and hand characteristics.
Polyolefins are available commercially in a wide range of forms. In general, the polymer properties are determined by the average molecular weight of the polyolefin and by the distribution of the various molecular weight fractions within the resin. High molecular weight polyolefin resins in general have a low melt flow rate (MFR) which is a measure of the amount of polymer which can be forced through a given sized orifice at a given temperature. Conversely, low molecular weight polyolefin resins generally have a high MFR. Because of the need for rapid attenuation during the spinning and drawdown process, relatively low molecular weight polyolefin resins are typically employed in meltspinning and typically have an MFR of from 20-50 as measured by ASTM D-1238-82, condition 230/2.16.
Polypropylene is commercially available in two principal grades. The first grade is generally known as CR (Controlled Rheology) grade. Polypropylene of this grade generally has a narrow molecular weight distribution as a result of a visbreaking treatment of the polymer recovered from the polymerization zone. The second and lower grade of polypropylene is generally known as Reactor Grade. This polypropylene generally has a broad molecular weight distribution and has not been subjected to visbreaking. As a result, this material typically undergoes thermal degradation during melt-pelleting or melt-spinning.
Because of physical requirements imposed by the melt-spinning process, manufacturers are generally limited in their choices of polyolefin polymer for meltspinning of high quality and relatively fine denier filaments. As indicated above, such polyolefin resins. are generally CR grade resins having an MFR of between about 20 and about 50.
In practice there are substantial limitations on increasing spinning productivity. Specifically, increasing the polymer throughput while also increasing the drawdown force applied to the meltspun filaments generally increases process productivity. However, for any particular polymer there is generally a limit to the drawdown force which can be applied to the polymer without also producing an excess number of filament breakages. Although the ability of the polymer to withstand higher drawdown forces can be improved by moving to a higher molecular weight (MW) polymer or by using a broader molecular weight distribution (MWD) polymer, the higher MW or broader MWD polymers typically resist attenuation or drawdown due to high melt elasticity and can also exhibit a greater resistance to flow through the spinneret orifices. In pneumatic, hydraulic, centrifugal and gravitational drawing systems, high melt elasticity will also result in higher filament deniers, at equivalent drawing, forces and could also result in increasing the incidence of cohesive failure at elevated drawing force conditions. In either case, the spinning process is harmed and thus "spinnability" is compromised. Conversely, lowering the molecular weight of the polymer generally improves the flow of the polymer through the spinneret orifices but results in a limp spin-line which harms filament laydown and increases the incidence of filament collisions which in turn causes breaks and "marrier filaments", i.e., filaments which bond together on contact. Although the molecular weight distribution can also be narrowed, this results in filaments and fabrics with inferior properties. Specifically thermally bonded spunbond fabrics made with very low MWD polymers tend to exhibit low tensile properties. Thus, the polyolefin fiber producer is faced with practical limitations on improving productivity of the spinning process.